mvebu: gen_mvebu_sdcard_img.sh: use /bin/sh

There is nothing that needs bash anymore.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[add prefix to commit title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/mvebu/image/gen_mvebu_sdcard_img.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 #
 # Copyright (C) 2016 Josua Mayer
 #